In general, terephthalic acid is produced by liquid-phase oxidation reaction of p-phenylene group-containing compounds, e.g., p-dialkyl benzenes such as typically p-xylene, and then purified by various methods. The liquid-phase oxidation reaction is usually conducted in an acetic acid solvent in the presence of a catalyst such as cobalt and manganese or the catalyst and a promoter such as a bromine compound and acetaldehyde. As the method of purifying crude terephthalic acid produced by the liquid-phase oxidation reaction, there are known various methods such as the method of dissolving the crude terephthalic acid in acetic acid, water or a mixed solvent thereof under high-temperature and high-pressure conditions and then subjecting the resultant solution to catalytic hydrogenation treatment, decarbonylation treatment, oxidation treatment or recrystallization treatment, and the method of subjecting a slurry containing terephthalic acid crystals partially dissolved therein to a high-temperature immersion treatment.
The terephthalic acid may be purified by combination of a plurality of these treatments. For example, a solution of terephthalic acid or a slurry containing terephthalic acid partially precipitated which has been purified by the catalytic hydrogenation treatment, etc., is further subjected to crystallization treatment to obtain a high-purity terephthalic acid.
In the crystallization treatment, flush evaporation of a solvent may be used in some cases. In this case, a low-pressure and low-temperature slurry obtained by flush evaporation of the solvent is subjected to solid-liquid separation to obtain a high-purity terephthalic acid. In addition, the crystallization treatment may also be conducted by a multistage crystallization method. In this case, a plurality of crystallization vessels may be arranged in series, and connected to each other through a delivery conduit provided with a control valve for controlling a flow rate of the slurry fed therethrough.
In the treatments such as crystallization treatment in which a slurry is handled and treated, it is important to control a solubility of the terephthalic acid. More specifically, in order to improve a yield of the terephthalic acid, it is important to use the conditions in which crystals thereof are readily precipitated. However, on the other hand, it is often undesirable that the crystals are precipitated outside of the crystallization vessels. For this reason, as to the treatment steps in which such a slurry is handled and treated, there are known various techniques for preventing precipitation of the crystals. For example, in JP 2004-315456A, there is disclosed the technique in which a slurry treating apparatus disposed in a crystallization step of a process for producing a high-purity terephthalic acid, is charged with a sealing solution having a temperature ranging from a temperature lower by 30° C. than a slurry temperature in the apparatus (slurry temperature−(minus) 30° C.) to the slurry temperature. In the technique described in JP 2004-315456A, the crystals are prevented form being deposited onto slurry treating equipments which includes meters such as a level meter, a pressure gauge and a flow meter, valves such as a control valve, a stirrer bearing, and a equipments disposed on a slurry withdrawal line such as a pump, etc. Also, in JP 2000-86577A, there is disclosed the process for producing a high-purity terephthalic acid in which the terephthalic acid is crystallized by adding a large amount of low-temperature water to an aqueous solution obtained after a hydrogenation treatment to reduce the temperature of the aqueous solution. In the technique described in JP 2000-86577A, precipitation or deposition of the crystals is prevented, and handling of the crystals upon transportation, etc., can be facilitated.
Especially in the multi-stage crystallization process, it is important to control a solubility of the terephthalic acid, and there is known such a problem that a delivery conduit suffers from clogging owing to deposition of the precipitated terephthalic acid crystals. This is because a high-pressure and high-temperature slurry containing terephthalic acid undergoes decrease in pressure and temperature when flowing through the delivery conduit. The precipitated terephthalic acid crystals are deposited onto an inner wall surface of the delivery conduit, thereby narrowing a flow path of the slurry and finally clogging the flow path.
The above problem concerning clogging of the delivery conduit may be solved by interrupting the crystallization step and cleaning an inside of the delivery conduit. However, this method still has problems concerning a stability of yield or quality of the products. Further, since precipitation of the crystals is caused in the delivery conduit disposed between the crystallization vessels, the method of preventing clogging of the flow path simply by dissolving the terephthalic acid crystals deposited onto the inner wall surface of the delivery conduit tends to reduce a yield of a high-purity terephthalic acid and, therefore, is disadvantageous in view of productivity thereof. Therefore, in order to not only prevent clogging of the delivery conduit but also achieve stable yield or quality of the products, the technique described in JP 2004-315456A which relates to such a technique for preventing precipitation of the crystals at a position other than the delivery conduit, and the technique described in JP 2000-86577A using a large amount of water are both unsatisfactory. Therefore, there is a demand for developing further effective techniques.